1. Field of the Invention
This invention relates to apparatus and methods for mechanical properties testing and more particularly relates to an apparatus and method for maintaining a constant temperature across an electrically conductive specimen required to be mechanically tested.
2. Description of the Related Art
The development of new materials, for example in the aircraft industry, often requires the mechanical testing of large quantities of specimens at elevated temperatures. The enhancement of laboratory capabilities for more efficient testing of materials is imperative for progress in materials development.
Mechanical properties testing, including the testing for properties such as compression, tension and fatigue should be accomplished at a constant temperature within a defined gage length of the specimen. This testing establishes material characteristics such as ultimate tensile strength, yield strength and tensile elongation.
Traditionally, ovens and furnaces have been used for heating specimens for mechanical properties testing. Although these units perform satisfactorily, they have limited ability to reach the desired temperature within a short time. As a result of the size of the oven and the type of heating controls used up to several hours may elapse before elevated stable temperatures are realized. With several thousand specimens to be tested, this becomes a time consuming approach, resulting in an unacceptable cost.
Other methods for obtaining the desired temperature for mechanical properties testing have included the use of inductive heating sources. For example, U.S. Pat. No. 4,812,052, entitled "Apparatus For Creep Endurance Testing Structural Components", issued to Adam, et al, discloses use of induction heating for testing purposes. Similarly, U.S. Pat. No. 4,618,267, entitled "Remote Temperature-Set-Point Controller", issued to Burke, et al, discloses another device for carrying out mechanical strain tests using induction heating.
Use of induction heating produces a temperature drop during specimen elongation and, depending on the type of equipment used, requires relatively high power (i.e., approximately 2000 watts). Furthermore, use of induction heating requires a special coil design to obtain a constant temperature over a wide gage length (i.e. approximately two inches). In these cases, the coil windings must be specially spaced.
Still other methods for obtaining the desired temperature for mechanical properties testing have included the use of resistive heating sources. For example, U.S. Pat. No. 2,685,195, entitled "Fatigue Testing Machine", issued to L. A. Streblow, discloses a testing machine which uses resistive heating for testing purposes. Use of such resistive heating techniques, such as that disclosed in the Streblow reference, is generally undesirable because when the specimen is fractured, a larger arc results which is unsafe. Furthermore, undesirable hot spots develop.
U.S. Pat. No. 3,100,253, entitled "Temperature Control System for Specimen Testing", issued to W. J. O'Connor resolved one aspect of the problem with resistance heating techniques by utilization of a dummy load for arc suppression. However, even with arc suppression resistive heating techniques are deficient because arc suppression requires the interruption of the primary and secondary circuits of the transformer. This cannot be accomplished without temperature control penalties.
Therefore, it is a principle object of the present invention to provide a reliable, efficient method and apparatus for maintaining a constant temperature across a specimen being mechanically tested.
Another object of the present invention is to provide closed loop temperature control.